1. Field of the Invention
This invention relates to valves and is directed more particularly to a valve stem sealing means for prevention of fugitive emissions.
2. Description of the Prior Art
It is well known in the valve art to provide a valve stem with sealing means to prevent escape of fluid from the valve flow passageway, around the stem and into the atmosphere. Generally, such valve stem seals have proven effective in preventing undue leakage past the valve stem.
However, in view of environmental concerns, it has become important to ensure against leakage that in other times might have been viewed as acceptable, particularly when environmentally damaging materials or substances are being flowed through the valve.
The United States Environmental Protection Agency (EPA), acting under The Clean Air Act, has mandated a reduction of emissions in various plants and refineries, including stack emissions and "fugitive emissions". In contrast to stack emissions, which emanate from obvious point sources, fugitive emissions emanate from a variety of nonpoint sources, primarily in the form of leakage, including leakage from valve stems.
In some applications, it is beneficial to have "back-up" sealing assemblies, that is, a sealing assembly which comes into play only upon failure of another sealing assembly. Even where no leakage has been found from "standard" valves, operators are able to receive more "credit" from the EPA by substituting for their standard valves, valves with double stem seals.
Further, it is deemed desirable to provide sets of sealing assemblies in which the seals are tightened by different means, as by one seal assembly responding automatically to changing conditions within the valve and another seal assembly responding to manual operation, or a condition different from that triggering the first seal assembly.